Remove extraneous "Error: " from ResolutionError messages
PTFM-8631: Add tests and refactoring for resolution errors

This is for resolving to either a project or one or more data objects
Remove unused imports.
Eliminate some coverage report hackery.

Make coverage files go into a path specified absolutely (previously it
was just .coverage.FOO in the cwd of the process), so subprocesses don't
need to either (1) run in a specific cwd or (2) move their coverage
files back when they're done.
Remove unused import.
Add tests for dxpy.search.find_data_objects.
App builder: Add hint about network access with execDepends and tests
Specific test teardown code doesn't belong in common DXTestCase.
Fix tests (set folder='/' to search current proj only).